Coin operated telephones are subject to a variety of possible operating failures. Many of these failures are intermittent and hard to detect and as a result, considerable loss of revenue is incurred by the telephone companies because of the difficulty in detecting a failure or detecting the exact cause of a failure.
As a general rule, coin operated telephones are tested using a conventional ohmmeter and voltmeter. As a result it is necessary for maintenance personnel to operate the telephone set, monitoring the ohmmeter and voltmeter to determine whether or not proper voltage and ground signals appear on the line during the various operating cycles of the telephone set.
The problem is complicated by the fact that one must detect correct line voltage, correct line voltage polarity, correct off-hook line voltage, coin present conditions, correct coin collect voltage, correct coin return voltages, and lack of proper ground connection.